The present invention generally relates to optoelectronic devices and more particularly relates to high speed photodetectors having integrated electrical components.
High-speed optical receivers use a photodetector to convert a received optical signal to an output current signal. The output a current signal is typically coupled to the input of a transimpedance amplifier (TIA) that converts the current signal to a voltage signal.
Optical receivers and monitors using high speed photodetectors and TIAs typically integrate an external capacitor between the power supply and ground. The external capacitor substantially reduces the reception of power supply transients by the TIA that might otherwise be received and amplified into a noise contribution to the output signal. However, the output of the photodetector is typically wire bonded to the input of the external TIA. The wire bonds introduce a parasitic inductance that tends to degrade the signal generated by the photodetector.
In addition the return path for this signal is from the TIA ground through wire bonds to the circuit board, then through traces on the circuit board, the external capacitor and additional wire bonds, to the photodetector. This path also introduces parasitic inductance, which may also degrade the signal out of the transimpedance amplifier.
In one aspect of the present invention a photodetector includes an AC ground capacitively coupled to a bias terminal of the photodetector. The present invention therefore, capacitively couples the power supply voltage Vcc to ground at the point where the unamplified current signal generated by the photodetector is passed to a TIA. Advantageously, the described exemplary on chip capacitor design eliminates the inductance of external circuit traces between the power supply and an external capacitor.
The present invention is not limited to a particular photodetector. Rather the present invention may be used with a plurality of photodetectors such as for example an MSM (Metal-Semiconductor-Metal photodetector), avalanche photodiode or PIN photodiode.
In a further aspect of the present invention an opto-electronic device includes a photodetector having a capacitor formed monolithically on the same substrate as the photodetector, wherein the capacitor is coupled between the photodetector AC ground and the photodetector bias terminal.
In another aspect of the present invention an opto-electronic device includes a monolithic bias resistor formed between the cathode of the photodetector and the bias terminal.
In a still further aspect of the present invention an opto-electronic device includes an array of photodetectors, wherein two or more of the photodetectors have a monolithic capacitor(s) formed on the same substrate as the photodetector that capacitively couples the photodetectors"" AC grounds to a bias terminal of the array, and/or a monolithic bias resistor(s) formed between each cathode of the photodetector and a common array bias terminal.
It is understood that other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein are shown and described only embodiments of the invention by way of illustration of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive.